There is a recognized need for a safe, ecologically sound, reliable flameless heater, especially for foods. For campers, such flameless heating avoids the risks of a campfire. In military maneuvers, the avoidance of visible detection by an enemy is enhanced by the use of flameless heat.
One class of heating devices that meets such a need is based on short-circuited electrochemical cells (Gilman and Bramhall, 1978). In these devices, anode and cathode compartment reactions of actual or theoretical electrochemical cells are combined together, capturing as heat the energy from the overall net cell reaction. Such devices have been patented (Epstein, 1970; Kober, 1978), but difficulties with manufacturing procedures, poor storage capability, and a lack of flexibility in matching heater configuration and geometry with, for example, food container geometry have limited their use. An electrochemical heater based on the magnesium-water reaction that eliminates these problems has been patented (Kuhn et al., 1983) and applied with success; for example, as a flameless ration heater (FRH) used by the U.S. Army.
A talk co-authored by K. Kustin, W. Roberts, and I. A. Taub, and presented by K. Kustin at the American Chemical Society National Meeting in New York on Aug. 30, 1991 dealt with the mechanism of the iron-promoted magnesium-water reaction. The theme of this presentation was to explain the roles of iron and chloride ions in relation to the protective nature of the magnesium hydroxide surface coating. Two aspects of the dynamics of dihydrogen gas generation were mentioned. First was that its evolution fits first-order kinetics. Second was that its rate constant was close but not equal to the first-order temperature-evolution rate constant. It was not discussed or presented during this presentation that there were scavengeable molecular hydrogen precursors in solution.
The Kuhn et al. (1983) device uses the iron-promoted reaction between magnesium metal and water that is catalyzed by certain dissolved electrolytes, for example, NaCl. The combination of magnesium and iron, referred to as a "super corroding metallic alloy" consists of a mixture approximately five moles of iron to ninety-five moles of magnesium (5:95 iron:magnesium) blended by a metallurgical milling technique. A drawback of the device is that, simultaneously with generation of heat, flammable dihydrogen gas is generated.
In their invention, Kuhn et al. (1983) considered the hazardous aspects of H.sub.2 evolution and demonstrated that, for small packages, the gas could not be ignited with an open flame and spark. The device was therefore judged to be safe. Kuhn et al. (1983) admitted, however, that the generation of dihydrogen can be a problem under certain circumstances. They suggested that the gas, once formed, could be removed catalytically, for example, in the presence of a palladium-based catalyst by the reaction: EQU 2H.sub.2 +O.sub.2 =2H.sub.2 O+Heat
It should be noted that the Kuhn device does not teach the suppression of hydrogen formation, but attacks the problem of undesired hydrogen by removing it with additional reactions after its formation.
It is appropriate to consider the practicality of this suggestion for a typical application in which 60 ml of water has been added to a heater pad. As a result of the magnesium-water reaction, after the brief (5-10 min) heating period, the remaining solution is warmed, typically, to 80.degree. C. From tables of solubility data (Linke, 1956), the maximum concentrations of the two gases at this temperature would be approximately 8.times.10.sup.-4 M dissolved O.sub.2 and 4.times.10.sup.-4 M dissolved H.sub.2 at 1 atmosphere (atm) total pressure. The total pressure in contact with the heated solution would be due almost entirely to water vapor and H.sub.2 gas. Therefore, the estimated dissolved concentration is reasonable for dihydrogen. Dissolved dioxygen, however, is in contact with very little O.sub.2 gas, and its concentration would be expected to be one or two orders of magnitude lower than the estimate derived from tabulated solubility data.
Based on this assessment of conditions in the heater subsequent to addition of water, it is clear that at least two considerations make the proposed catalytic suppression of dihydrogen evolution as in Kuhn, et al, impractical. First is that insufficient dissolved O.sub.2 is present in the reaction mixture to combine with and remove H.sub.2. Catalytic action therefore requires dissolution of atmospheric dioxygen to react with the evolving H.sub.2. Mass transport of O.sub.2 from the atmosphere is not rapid (Patwardhan and Sharma, 1988) and is unlikely to raise the dioxygen concentration to a value sufficiently high to remove dihydrogen formed during the relatively short heating period. Second is that catalysis by palladium or silver is not rapid (Engel and Kuipers, 1979). Little combination of H.sub.2 with O.sub.2 would be expected to occur during the heating period through the agency of such catalysts.
It can be seen that there presently is not a way to provide such chemical heating that does not, to some degree, expose the surrounding atmosphere to free dihydrogen gas. It would be helpful to devise a method to suppress H.sub.2 formation that could be applied to a variety of heating applications. In particular, it would be helpful to devise a means of accomplishing such chemical heating that could be applied to food in quantities sufficient to feed from one to forty or so individuals as may be appropriate in military field environments.